1. Field of the Invention
The present invention relates to image forming apparatuses and, more particularly, to an image forming apparatus that adjusts an amount of reference exposure so that an exposure unit performs image formation properly.
2. Description of the Related Art
Image forming apparatus that performs a process control is widely known. In the process control, an image forming apparatus disclosed in Japanese Patent Application Laid-open No. H5-14729 forms a toner pattern on a photosensitive element under predetermined conditions in the same manner as a normal toner image is formed. It then uses a sensor to detect the amount of toner forming the toner pattern and adjusts various parameters based on the result of the detection, such as charging bias, developing bias, and the amount of reference exposure (hereinafter, “reference exposure amount”). These actions are performed so that a desirable image can be formed. In such an image forming apparatus, the relations are determined between development potential and various parameters, such as the charging bias, the developing bias, and the reference exposure amount, (hereinafter, “the various parameters”). These relations are determined using the results of experiments or the like. A data table including the development potential and the corresponding various parameters is stored in a storage device in the image forming apparatus. In the process control, the image forming apparatus acquires the result detected by the sensor, calculates the development potential based on the result, identifies the various parameters corresponding to the calculated development potential by referring to the data table, and sets identified various parameters. In some image forming apparatuses, the development potential is determined using another parameter, such as a development gamma instead of the various parameters mentioned above.
Once the development potential is calculated based on the result detected by the sensor, the image forming apparatus can adjust the various parameters to suitable values by referring only to the data table. The simplicity with which adjustments can be made is an advantage of the image forming apparatuses using the data table.
The simple adjustment system using the data table can work properly only when the relations between the development potential and the various parameters are not significantly affected by the passage of time and changes in the environment. If the relations between the development potential and the various parameters are affected significantly by the passage of time or changes in the environment, it is necessary to store additional data tables for expected changes due to the passage of time and changes in the environment. This increases the amount of data required, which means that the simple adjustment system using the data tables becomes less attractive.
Photosensitive elements are widely known that have a high wear resistance and a long operating life. The photosensitive elements have a surface layer containing a filler that gives the elements their high wear resistance.
According to studies by the inventors, it has been confirmed that, if a longer-lasting photosensitive element is used, the relation between the amount of exposure (hereinafter, “exposure amount”) that is used for image formation and the potential of an exposed area (hereinafter, “exposed-area potential VL”) changes significantly with the passage of time and because of changes in the environment.
The exposure amount, hereinafter, is calculated by multiplying the power of a light source (hereinafter, “exposure power”) per unit area on the surface of the photosensitive element (e.g., area corresponding to a 1-dot electrostatic latent image) by an exposure time (hereinafter, “unit exposure time”).
FIG. 14A is a graph for explaining the relation between the exposure power of a laser diode (LD) and the exposed-area potential VL that is observed after image formation is performed at predetermined times in a high-temperature and high-humidity environment. FIG. 14B is a graph for explaining the relation between the exposure power of the LD and the exposed-area potential VL that is observed when image formation is performed at the predetermined times the same as in the example illustrated in FIG. 14A in low-temperature and low-humidity environment. The unit exposure time is set to the longest values available for image formation. It is clear from the graphs that the relation between the exposure power that is used for image formation (ranging from the left side to around the center of each of the graphs in FIGS. 14A and 14B) and the exposed-area potential VL changes significantly depending on the differences in the environments. Therefore, if a photosensitive element is used in which the relation between the exposure amount that is used for image formation and the exposed-area potential VL can change significantly (hereinafter, “specific photosensitive element”), the relation between the exposure power that is used for image formation and the exposed-area potential VL significantly changes due to the changes in the environment.
The difference in the relation between the exposure amount that is used for image formation and the exposed-area potential VL can be expressed by the difference in the potential of the exposed area that is detected when the surface of the photosensitive element is exposed by the LD at the maximum exposure amount. In other words, the difference in the relation between the exposure amount and the exposed-area potential VL can be calculated using the difference in residual potential that remains on the surface of the photosensitive element after the exposure unit irradiates the photosensitive element with light to discharge electricity, with the irradiated light being at the maximum exposure amount (hereinafter, “residual exposed-area potential Vr”). The exposed-area potential VL corresponding to the maximum exposure power, i.e., the exposed-area potential VL furthest to the right in each of FIGS. 14A and 14B, corresponds to the residual exposed-area potential Vr. It is clear from FIGS. 14A and 14B that the residual exposed-area potential Vr in the high-temperature high-humidity environment is low, while the residual exposed-area potential Vr in the low-temperature low-humidity environment is high. That is, the residual exposed-area potential Vr is affected significantly by the environment. The difference in the residual exposed-area potential Vr correlates to the difference in the relation between the exposure power that is used for image formation and the exposed-area potential VL.
FIG. 15 is a graph for explaining the relation between the number of sheets and the residual exposed-area potential Vr in an image forming apparatus that includes a specific photosensitive element.
It is clear from the graph that the residual exposed-area potential Vr changes significantly with the passage of time. Therefore, if a specific photosensitive element is used, the relation between the exposure power that is used for image formation and the exposed-area potential VL changes significantly with the passage of time.
In the image forming apparatus that includes a specific photosensitive element in which the relation between the exposure power that is used for image formation and the exposed-area potential VL can change significantly, the relations between parameters including the development potential and the reference exposure amount also change significantly. This is because the suitable exposed-area potential VL is determined by the development potential that is calculated based on the result detected by the sensor in the process control; nevertheless, the exposure power corresponding to the suitable exposed-area potential VL changes significantly with the passage of time or due to changes in the environment. Therefore, if the reference exposure amount suitable for acquiring the target exposure power is determined by referring to the data table in the same manner as in conventional image forming apparatus, disadvantages, such as an increase in the amount of required data, will outweigh the advantages.